As is known in the art, CT is a method to make full use of acquired information and image to diagnose diseases, which maintains a certain field of view (FOV) through one examination. However, there is chance that the imaged subject exceeds the FOV, which is generally referred to in the art as a truncation. FIG. 1A illustrates a normal scan scenario and three truncation scenarios, which are respectively shown in FIG. 1B, FIG. 1C, and FIG. 1D.
If a truncation occurs, a bright external ring having roughly a width of twenty pixels sometimes appears in the reconstructed image, as shown in FIG. 2A. FIG. 2A shows an image from a radiograph for a chest, in which a bright external ring occurs due to applying zero padding to an original data of the truncated region in filtering for image construction. Although the bright ring gives hint of the truncated region, it loses real edge image information and thus degrades the image quality.
Typically, conventional CT systems extrapolate truncated projection data to reduce image discontinuity artifacts. Generally, two approaches are widely used in the art. One is to apply a nearest padding to the projection data of the truncated region in filtering for image construction, as shown in FIG. 2B. The other approach is mirror padding, which mirrors the previous projection data to the truncated region in filtering for image construction, as shown in FIG. 2C. However, it is hard to accurately predict the data trend of the truncated region in actual practice due to the complicated structure of a human body. Consequently, the image processed using either of these two approaches is not real with respect to the presentation of the truncated region. However, the presentation characteristics of the image, for example, the pixel brightness, are always used by a physician in actual practice to diagnose. Unfortunately, due to unreal presentation, the truncated region obtained with the aforesaid two approaches cannot be used for diagnosis purpose. Therefore, a physician needs to know which regions of the image are truncated so that the truncated regions will not be used to incur a misdiagnosis. Although the aforesaid two approaches increase the image quality around the truncated region, they lose the bright rings as well. In the absence of the bright rings, a physician will not be able to determine which regions are truncated when reviewing an image. Most of the physicians have doubts to the nearest padding scenario, the mirror padding scenario, and the off-center reconstruction, because they have no way to know which region or location of the image is destroyed by truncation. Besides, most of the conventional CT methods aim at correcting truncation. For example, U.S. Pat. No. 7,254,259, titled “Method and Apparatus for compensating truncation”, which is incorporated herein by reference, merely teaches how to compensate truncation. In view of above, the physicians remain unable to know which regions are truncated while reviewing an image.